Angularly fixed longitudinal supports such as plates and bars increasingly are used in osteosynthesis applications. Such devices are particularly useful for treating fractures that are located near joints, or for anchoring screws in the spinal column. In applications that use short screws, the screws typically can be inserted in the longitudinal support at a preset angle without presenting problems. When longer screws are necessary, a fixed, system-dependent orientation of the screw may be impractical or unwieldy.
To facilitate the use of longer screws in regions such as the spine, special ball joints have been developed. In addition, as disclosed in German patent DE 195 48 395, bone plates have been proposed with specially configured screw holes drilled therein for accepting correspondingly shaped screw heads. The bone screw thus may be locked in place in the bone plate in a relatively randomly selectable orientation. But, the complexity, bulky nature, and insufficient strength provided by the connections of these ball joints and screw head-borehole configurations does not sufficiently remedy the inherent problems encountered with fixed, system-dependent orientations of screws.
Also disclosed in German patent DE 24 38 669 to Bezold is an osteosynthesis bone plate with screw holes having a respective spacing that can be manipulated using externally generated forces. The screw holes are arranged in the form of lugs punched out of the main body of the osteosynthesis plate and connected thereto merely by elastic legs. In one embodiment, the legs are aligned along one axis and connected to the lug diametrically relative to the axis. When the lug is lifted out of the plane of the plate, the legs are bent such that the rotational axis of the lug no longer coincides with the vertical axis of either the lug itself or the screw hole. Use in clinical applications thus is limited because as the lug is turned, the center of the screw hole is undesirably shifted.
There exists a need for a bone plate with a coupling that facilitates the use of a wide range of sizes of bone screws. There further exists a need for a coupling that requires less space than ball joints and provides simplicity in design and use. Additionally, there is a need for a substantially flat swivel joint for use in connecting implant components such as a bone screw and a bone plate.
The present invention provides an implant and coupling capable of furnishing these improvements, and advantageously has application in a wide range of other implants unrelated to bone screw support.